1. Field of the Invention
The present invention relates to an optical device which leads light beams emitted from a light source to a predetermined illumination area, an illumination apparatus which illuminates a predetermined illumination target surface by using such an optical device, and a projector which projects an image by using such an optical device.
2. Description of the Related Art
Various kinds of optical devices which lead light beams emitted from a light source to a predetermined illumination area have been conventionally proposed.
For example, U.S. Pat. No. 5,829,858 discloses an optical device which leads light beams from a light source which have entered an incident surface, and allows them to be emitted from an outgoing radiation surface. This optical device is used to illuminate a rectangular illumination area in cases where a lamp which emits light beams with a circular light distribution with respect to the incident surface is a light source. Therefore, this optical device has a structure that the incident surface has a circular or octagonal shape and the outgoing radiation surface has a rectangular shape.
Further, U.S. Pat. No. 6,139,156 also discloses a tapered optical device having a structure that an incident surface has a circular shape and an outgoing radiation surface has a rectangular shape.
On the other hand, a light emitting diode (which will be abbreviated as an LED hereinafter) has recently wined considerable attention as a small light source for coming generation. The LED has advantages such as smallness in size, resistance characteristics, a long duration of life or the like. However, because of a restriction in its light emission efficiency and light emission output, the conventional LED is mainly used as an indicator illumination lamp for various gauges and the like or a control state confirmation lamp. However, in recent years, the light emission efficiency of the LED has been rapidly improved, and it is said that it is a matter of time before this light emission efficiency exceeds the light emission efficiency of a high pressure mercury lamp or a fluorescent lamp which has been conventionally considered as a discharge type lamp with the highest light emission efficiency.